As described in US20130170687A1 and U.S. Pat. No. 8,794,373B1 the application of an air-adsorbing structure in the closed cabinet (back volume) of a loudspeaker can significantly enhance the emission in the low frequency sound range. Molded paddings like granules or monoliths, consisting of hydrophobic zeolites with void spaces of a specific porosity have shown to be an especially suitable material.
The materials described in US20130170687A1 and U.S. Pat. No. 8,794,373B1 have several disadvantages. The 3D monolithic structure described in U.S. Pat. No. 8,687,836B1 has to be adapted to the geometry of the closed cabinet. This adaption might not be feasible, especially in the case of small and complex back volume geometries. Such small and complex back volume geometries are often found in mobile devices. It is thus more advantageous to fill these geometries with a granular material, such as that described in US20130170687A1 to be more flexible. However, irregular shaped granules with sharp edges possess a low flowability and can produce dust due to abrasion as e.g. described in WO1995018675A1.
In contrast to such irregular formed granules, spheres have a number of advantages. The spheres exhibit a higher degree of mechanical robustness, as described in (Krasii, B. V., V. B. Maryshev, and T. S. Kustova. “Comparison of mechanical strength of reforming catalysts of different geometry.” Catalysis in Industry 1.4 (2009): 364-366. or US20100196213). The spheres do not contain sharp edges, which can lead to abrasion and dust formation as described in (ASTM D 4058-87, “Standard Test for Attrition and Abrasion of Catalysts and Catalyst Carriers”) and show an enhanced flowability (no congestion during dosing—see e.g. U.S. Pat. No. 4,687,672). Furthermore, with monodisperse spheres, reproducible and dense packings with defined void spaces of a given volume are possible. In particular the ability of the spheres to fill irregular formed geometries is advantageous.
In the loudspeakers used in mobile devices, attrition and dust formation should be avoided. The dust of any inorganic material, such as zeolites, might be inhaled by the user of the mobile device and can be harmful to health. Such dust formation might not only be caused by the internal mechanical stress during regular operation of a loudspeaker, but also by external shocks and vibrations.
It will be understood therefore that there is a need for spherical particles with enhanced flowability for filling complex back volumes of a loudspeaker device, resulting in a mechanical robust, dust preventing and dense padding.
The disclosure teaches the synthesis of highly spherical particles with a defined porosity and a high abrasion resistance for use in loudspeakers.